Negative electrode active material for lithium secondary battery and method for preparing the same

ABSTRACT

A negative electrode active material for a lithium secondary battery including complex particles including silicon oxide particles doped with a metal of lithium, magnesium, calcium or aluminum, a linear conductive material disposed between the doped silicon oxide particles, and a carbon-based binder that hinds the doped silicon oxide particles and the conductive material together. The carbon-based binder is a result of sintering. a carbon-based precursor. A method of preparing the negative electrode active material, a negative electrode and a lithium secondary battery are also provided. The negative electrode active material according to the present disclosure is superior at least in initial efficiency and life characteristics.

TECHNICAL FIELD

The present application claims the benefit of Korean Patent ApplicationNo. 10-2018-0059234 filed on May 24, 2018 with the Korean IntellectualProperty Office, the disclosure of which is incorporated herein byreference in its entirety.

The present disclosure relates to a negative electrode active materialfor a lithium secondary battery and a method for preparing the same, andmore particularly, to a negative electrode active material for a lithiumsecondary battery with high initial efficiency and long lifecharacteristics and a method for preparing the same.

BACKGROUND ART

With the technology development and growing demand for mobile devices,the demand for secondary batteries as an energy source dramaticallyincreases. In secondary batteries, lithium secondary batteries arewidely used in commercial applications due to their high energy densityand voltage, long cycle life, and low discharge rate.

A lithium secondary battery has a structure in which an electrodeassembly including a positive electrode and a negative electrode, eachhaving an active material applied on an electrode current collector,with a porous separator interposed between, is impregnated with anelectrolyte containing a lithium salt, and the electrode is manufacturedby applying, to a current collector, a slurry containing an activematerial, a binder and a conductive material dispersed in a solvent,followed by drying and pressing.

Additionally, the basic performance characteristics of the lithiumsecondary battery, namely, capacity, output and life, are greatlyinfluenced by the negative electrode material. For maximum batteryperformance, the negative electrode active material needs to satisfy therequirements that the electrochemical reaction potential should be closeto lithium metal, reaction reversibility with lithium ions should behigh and diffusion of lithium ions in the active material should befast, and a carbon-based material is widely used as a materialsatisfying these requirements.

The carbon-based active material is good at stability and reversibility,but has capacity limitation. Accordingly, recently, Si-based materialshaving high theoretical capacity are applied as a negative electrodeactive material in the field of industry requiring high capacitybatteries, for example, electric vehicles and hybrid electric vehicles.

However, Si particles change in crystal structure when lithium ions areintercalated during charging, and involve volume expansion such that thevolume is about 4 times larger than that before lithium intercalation.Accordingly, Si particles do not withstand the volume changes in therepeated charging/discharging and are cracked in the crystals andbroken, electrical connection between adjacent particles reduces, andeventually the life characteristics degrade.

Accordingly, studies have been made to improve the life characteristicsusing silicon oxide (SiO_(x)) and reduce the volume expansion, butbecause silicon oxide forms an irreversible phase when lithium isintercalated, as lithium is consumed, the initial efficiency is low andthe life characteristics degrade.

DISCLOSURE Technical Problem

The present disclosure is designed to solve the above-described problem,and therefore the present disclosure is directed to providing a siliconoxide-based negative electrode active material with improved initialefficiency and life characteristics and a method for preparing the same.

The present disclosure is further directed to providing a negativeelectrode including the negative electrode active material and a lithiumsecondary battery including the same.

Technical Solution

According to an aspect of the present disclosure, there is provided anegative electrode active material for a lithium secondary batteryincluding complex particles including silicon oxide particles doped withat least one metal of lithium, magnesium, calcium and aluminum, a linearconductive material disposed between the doped silicon oxide particles,and a carbon-based binder which binds the doped silicon oxide particlesand the conductive material together, wherein the carbon-based binder isa result of sintering a carbon-based precursor.

The silicon oxide may be represented by SiO_(x)(0<x≤2).

An amount of the metal doped in the silicon oxide may be 1 to 50 weight%.

The doped silicon oxide particles may have an average particle diameterof 1 to 6 μm.

The complex particles may have an average particle diameter of 3 to 12μm.

The carbon-based precursor may be pitch.

The linear conductive material may be carbon nanotube (CNT), graphene orcarbon black.

Each of the carbon-based binder and the linear conductive material maybe present in an amount of 1 to 30 parts by weight based on 100 parts byweight of the doped silicon oxide.

According to another aspect of the present disclosure, there is provideda method for preparing a negative electrode active material for alithium secondary battery, including mixing silicon oxide particles withmetal powder of lithium, magnesium, calcium or aluminum and, then,performing heat treatment to dope the silicon oxide particles; andmixing the doped silicon oxide particles with a carbon-based precursorand a linear conductive material, and, then, performing sintering toform the doped silicon oxide particles and the linear conductivematerial into a complex by the medium of a carbon-based binder as aresult of sintering the carbon-based precursor.

The heat treatment in the doping may be performed at 800 to 1,050° C.

The sintering in the forming into a complex may be performed at 700 to1,100° C.

According to still another aspect of the present disclosure, there isprovided a negative electrode including a current collector, and anegative electrode active material layer disposed on at least onesurface of the current collector, wherein the negative electrode activematerial layer includes the negative electrode active material for alithium secondary battery as mentioned above.

In addition, there is provided a lithium secondary battery including thenegative electrode.

Advantageous Effects

The negative electrode active material according to an aspect of thepresent disclosure includes doped silicon oxide, and a carbon-basedbinder binds particles of the doped silicon oxide and a linearconductive material between the particles together to minimize the gapscaused by the volume expansion of silicon oxide, and even though gapsare generated, the linear conductive material serves as a bridge fillingthe gaps, thereby enhancing the initial efficiency and lifecharacteristics.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings illustrate preferred embodiments of thepresent disclosure and, together with the foregoing disclosure, serve toprovide a further understanding of the technical aspects of the presentdisclosure. However, the present disclosure is not to be construed asbeing limited to the drawings.

FIG. 1 shows the structure of a negative electrode active material for alithium secondary battery according to an embodiment of the presentdisclosure.

BEST MODE

Hereinafter, it should be understood that the terms or words used in thespecification and the appended claims should not be construed as limitedto general and dictionary meanings, but interpreted based on themeanings and concepts corresponding to technical aspects of the presentdisclosure on the basis of the principle that the inventor is allowed todefine terms appropriately for the best explanation.

A negative electrode active material for a lithium secondary batteryaccording to an embodiment of the present disclosure includes complexparticles including doped silicon oxide, a linear conductive materialand a carbon-based binder, and FIG. 1 illustrates the structure of thenegative electrode active material for a lithium secondary battery ofthe present disclosure.

Referring to FIG. 1, the negative electrode active material 100 for alithium secondary battery of the present disclosure includes complexparticles in which doped silicon oxide 10 is in the form of secondaryparticles resulting from the aggregation of primary particles, thelinear conductive material 20 and the carbon-based binder 30 aredisposed between the particles of the doped silicon oxide 10, and thecarbon-based binder 30 binds the particles of the doped silicon oxide 10and the linear conductive material 20 together.

In an embodiment of the present disclosure, the doped silicon oxiderefers to silicon oxide doped with at least one metal of lithium,magnesium, calcium and aluminum to overcome the drawback of siliconoxide having low initial efficiency.

Silicon oxide generally represented by SiO_(x)(0<x≤2) includes ananocomposite structure of SiO₂ alone or a mixture of Si and SiO₂, andits composition x may be determined by a ratio of silicon and oxygen.For example, when Si:SiO₂ are mixed at a mole ratio of 1:1 in theSiO_(x)(0<x≤2), SiO (x=1) may be represented.

When silicon oxide of SiO_(x)(0<x≤2) is applied as a negative electrodeactive material of a lithium secondary battery, Si included in siliconoxide substantially causes electrochemical reactions by intercalationand deintercalation of lithium ions separated from a positive electrodeactive material in the charging reaction of the lithium secondarybattery. In this instance, irreversible reaction whereby lithiumcompounds not contributing to the charging/discharging are generated mayoccur in the initial charge. As a result, it is known that initialefficiency of the lithium secondary battery including silicon oxide asthe negative electrode active material is low.

To overcome this problem, the present disclosure uses silicon oxidedoped with at least one metal component of lithium, magnesium, calciumand aluminum. For example, when silicon oxide is doped with the metalcomponent, the metal is bonded to the oxygen-containing Si compound(SiO, SiO₂, SiO_(x)) part and causes irreversible reactions first. Inthis state, when charging is performed, lithium ions intercalated intothe negative electrode are bonded to Si, and when discharging isperformed, the lithium ions are deintercalated, and thus, the initialirreversible reactions of silicon oxide reduce, resulting in improvedinitial efficiency. Additionally, this improved initial efficiencyreduces an amount of lithium ions bonded to Si during discharging of afull cell and regulates the operating potential of the negativeelectrode, and eventually, reduces the volume change of Si, resulting inimproved life characteristics.

An amount of at least one of lithium, magnesium, calcium and aluminumdoped in the silicon oxide may be 1 to 50 weight %, particularly 2 to 30weight %, more particularly 3 to 20 weight % for appropriate initialefficiency and enhanced life characteristics.

In an embodiment of the present disclosure, the particles of the dopedsilicon oxide may have the average particle diameter of 1 to 6 μm. Forexample, the average particle diameter of the doped silicon oxideparticles may range between the lower limit and the upper limit, withthe lower limit being 1 μm or 1.5 μm, and the upper limit being 6 μm or3 μm. When the particle size is too small, side reactions increase, andwhen the particle size is too large, the particles may be cracked duringcharging/discharging, and thus it is advantageous when the particle sizesatisfies the above range. To control the particle size, pulverizationand sieving may be performed on the doped silicon oxide. In thisinstance, the average particle diameter of the particles may bedetermined by methods commonly used in the art, for example, laserdiffraction particle size distribution measurement.

In an embodiment of the present disclosure, the linear conductivematerial is used to overcome the problem that silicon particlescontained in silicon oxide cannot withstand a volume change ascharging/discharge repeats, cracks occur in the crystals, the particlesbreak, and electrical connection between adjacent particles reduces andlife characteristics degrade.

The linear conductive material refers to a conductive material such thata single material or a result of agglomeration has a large aspect ratio(length/diameter), for example, an aspect ratio of 50 to 500.Additionally, the linear conductive material may have an averagediameter of 1 to 200 nm and an average length of 100 nm to 5 μm. Thelinear conductive material is disposed between the doped silicon oxideparticles to give conductivity so that electrical connection between theparticles is maintained. And, the linear conductive material serves as abridge filling the gaps caused by the volume expansion of silicon oxideduring charging/discharging, contributing to the structure maintenanceof the complex particles, thereby improving the life characteristics.

The linear conductive material such that a single material has theaspect ratio may be carbon nanotube (CNT) and graphene, and the linearconductive material such that a result of agglomeration has the aspectratio may be carbon black.

Additionally, the linear conductive material may be present in an amountof 1 to 30, particularly 3 to 15 parts by weight based on 100 parts byweight of the doped silicon oxide. When the above range is satisfied, itis possible to establish sufficient electrical connection betweenadjacent particles and minimize side reactions with an electrolytesolution, thereby preventing the reduction in initial efficiency andlife characteristics.

In an embodiment of the present disclosure, the carbon-based binder isdisposed between the doped silicon oxide particles while being in pointcontact with the silicon oxide particles and the linear conductivematerial, and serves as a binder to bind them together into a complex.

That is, the complex particles of the negative electrode active materialaccording to the present disclosure have enhanced bonds between adjacentdoped silicon oxide particles by the carbon-based binder, therebyminimizing the gaps even though cracks occur due to volume expansion.Further, the linear conductive material is bonded with the silicon oxideparticles together at the same time, suppressing structural changes, andwhen gaps are created between the silicon oxide particles or in theparticles, the linear conductive material having a large aspect ratioconnects the gaps, thereby preventing the reduction in electricalconductivity, and leading to much more improved life characteristics.

The carbon-based binder may be a result of sintering a carbon-basedprecursor, and the carbon-based precursor may be pitch.

Additionally, the carbon-based binder may be present in an amount of 1to 30, particularly 3 to 15 parts by weight based on 100 parts by weightof the doped silicon oxide. When the above range is satisfied, it ispossible to give sufficient bondability and conductivity and prevent theinitial efficiency reduction.

In the present disclosure, the amount of the carbon-based binder may becalculated by subtracting the sum of the weight of the doped siliconoxide and the weight of the linear conductive material from the totalweight of the complex particles.

As described above, the complex particles of the present disclosure, inwhich the silicon oxide particles are bonded with the linear conductivematerial therebetween together by the medium of the carbon-based binder,may have the average particle diameter of 3 to 12 μm. For example, theaverage particle diameter of the complex particles may range between thelower limit and the upper limit, with the lower limit being 3 μm, 5 μmor 7 μm, and the upper limit being 12 μm or 10 μm.

The complex particles of the present disclosure are 2 to 5 times,particularly 2 to 4 times larger in size than the doped silicon oxideparticles. Here, the size ratio of the complex particles to the dopedsilicon oxide particles may be calculated by dividing the averageparticle diameter of the complex particles by the average particlediameter of the doped silicon oxide.

That is, the negative electrode active material according to anembodiment of the present disclosure include the doped silicon oxideparticles agglomerated on a small scale, not a large scale like bunch ofgrapes, and this is advantageous in terms of processing since thestructure is maintained even after coating and pressing.

Another embodiment of the present disclosure relates to a method forpreparing the negative electrode active material for a lithium secondarybattery as described above, and in detail, the method includes thefollowing steps:

(S1) mixing silicon oxide particles with metal powder of lithium,magnesium, calcium or aluminum and performing heat treatment to dope thesilicon oxide particles; and

(S2) mixing the doped silicon oxide particles with a linear conductivematerial and a carbon-based precursor and performing sintering.

In the step S1, heat treatment may be performed at 800 to 1050° C.,particularly 900 to 1000° C., taking into account the melting point andthe boiling point of the metal component used for silicon oxide doping.That is, the heat treatment temperature is selected as a suitabletemperature so that after silicon oxide is mixed with at least one metalpowder of lithium, magnesium, calcium and aluminum, the metal componentsdo not evaporate and can be dissolved and impregnated into the siliconoxide. Additionally, the heat treatment may be performed, for example,for 1 to 3 hours.

Meanwhile, an amount of at least one metal powder of lithium, magnesium,calcium and aluminum may be 1 to 50 weight %, particularly 2 to 30weight %, particularly 3 to 20 weight % relative to the silicon oxide toprovide a sufficient initial efficiency improvement effect without agreat reduction in discharge capacity.

The step S2 is a process in which the doped silicon oxide particles andthe linear conductive material are formed into a complex by thecarbon-based binder. That is, through the sintering process after mixingthe doped silicon oxide particles with the carbon-based precursor andthe linear conductive material, the carbon-based binder as a result ofsintering the carbon-based precursor binds the doped silicon oxideparticles and the linear conductive material to produce complexparticles.

In the step S2, the linear conductive material may be used in an amountof 1 to 30, particularly 3 to 15 parts by weight based on 100 parts byweight of the doped silicon oxide. Additionally, the carbon-basedprecursor may be used in an amount of 1 to 30, particularly 3 to 15parts by weight based on 100 parts by weight of the doped silicon oxide.

Meanwhile, the sintering may be performed at 700 to 1,100 ° C., forexample, 800 to 1,000 ° C. for 1 to 5 hours, taking into accountcrystallinity of the silicon oxide, the carbon-based binder and thelinear conductive material.

Still another embodiment of the present disclosure relates to a negativeelectrode including the negative electrode active material prepared asdescribed above.

In detail, the negative electrode according to an embodiment of thepresent disclosure includes a current collector, and a negativeelectrode active material layer including the negative electrode activematerial according to the present disclosure on at least one surface ofthe current collector.

The electrode layer may be formed by coating a slurry for a negativeelectrode active material layer obtained by dispersing the negativeelectrode active material according to the present disclosure, a binderand a conductive material in a solvent on at least one surface of thecurrent collector, followed by drying and pressing.

The current collector is not limited particularly if it causes nochemical change in the battery and has conductivity, and may include,for example, copper, stainless steel, aluminum, nickel, titanium,sintered carbon, copper or stainless steel surface-treated with carbon,nickel, titanium or silver, and aluminum-cadmium alloy. The thickness ofthe current collector is not particularly limited, but may have thethickness of 3˜500 μm as commonly applied.

The negative electrode active material shows high initial efficiencybecause of including the doped silicon oxide, and comprises complexparticles wherein the carbon-based binder binds the particles of thedoped silicon oxide and the linear conductive material between theparticles together to minimize the gaps caused by the volume expansionof silicon oxide, and even though gaps are generated, the linearconductive material serves as a bridge filling the gaps, therebycontributing to improvement of the life characteristics of a battery.

The negative electrode active material may be present in an amount of 80weight % to 99 weight % based on the total weight of the negativeelectrode slurry composition.

The binder is a component that aids bonding between the conductivematerial, and the active material, or the current collector, and isgenerally present in an amount of 0.1 to 20 weight % based on the totalweight of the negative electrode slurry composition. Examples of thebinder include polyvinylidene fluoride-co-hexafluoropropylene(PVDF-co-HEP), polyvinylidenefluoride, polyacrylonitrile,polymethylmethacrylate, polyvinylalcohol, carboxylmethylcellulose (CMC),starch, hydroxypropylcellulose, regenerated cellulose,polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene,polyacrylate and styrene butadiene rubber (SBR).

The conductive material is not limited to a particular type if it causesno chemical change in the corresponding battery and has conductivity,and may include, for example, carbon black such as carbon black,acetylene black, ketjen black, channel black, furnace black, lamp blackand thermal black; a conductive fiber, for example, a carbon fiber or ametal fiber; metal powder, for example, fluorocarbon, aluminum andnickel powder; conductive whisker, for example, zinc oxide, potassiumtitanate; conductive metal oxide, for example, titanium oxide; and aconductive material, for example, a polyphenylene derivative. Theconductive material may be added in an amount of 0.1 to 20 weight %based on the total weight of the negative electrode slurry composition.

The solvent may include an organic solvent, for example, water orN-methyl-2-pyrrolidone (NMP), and may be used in such an amount for thedesirable viscosity when the negative electrode slurry includes thenegative electrode active material, and optionally, the binder and theconductive material.

Additionally, the coating method of the negative electrode slurry is notlimited to a particular type, and includes any coating method commonlyused in the art. For example, a coating method using a slot die may beused, and besides, a Meyer bar coating method, a Gravure coating method,a dip coating method, a spray coating method, etc. may be used.

Yet still another embodiment of the present disclosure relates to alithium secondary battery including the negative electrode. In detail,the lithium secondary battery may be manufactured by injecting anelectrolyte containing a lithium salt into an electrode assemblyincluding a positive electrode, the negative electrode as describedabove, and a separator interposed between.

The positive electrode may be manufactured by mixing a positiveelectrode active material, a conductive material, a binder and a solventto prepare a slurry, and directly coating the slurry on a metal currentcollector, or laminating, on a metal current collector, a positiveelectrode active material film cast on a separate support and peeled offfrom the support.

The active material used for the positive electrode is active materialparticles of any one selected from the group consisting of LiCoO₂,LiNiO₂, LiMn₂O₄, LiCoPO₄, LiFePO₄ andLiNi_(1-x-y-z)Co_(x)M1_(y)M2_(z)O₂(M1 and M2 are independently any oneselected from the group consisting of Al, Ni, Co, Fe, Mn, V, Cr, Ti, W,Ta, Mg and Mo, and x, y and z are independently atomic fractions ofelements in the oxide composition, where 0≤x<0.5, 0≤y<0.5, 0≤z<0.5,0<x+y+z≤1), or their mixtures.

Meanwhile, the conductive material, the binder and the solvent may bethe same as those used in manufacturing the negative electrode.

The separator may include a general porous polymer film conventionallyused for separators, for example, a porous polymer film made of apolyolefin-based polymer such as ethylene homopolymer, propylenehomopolymer, ethylene/butene copolymer, ethylene/hexene copolymer andethylene/methacrylate copolymer used singly or in stack. Additionally,an insulating thin film having high ion permittivity and mechanicalstrength may be used. The separator may include a safety reinforcedseparator (SRS) having a thin coating of a ceramic material on theseparator surface. Further, a general porous nonwoven fabric, forexample, a nonwoven fabric made of high melting point glass fibers andpolyethyleneterephthalate fibers may be used, but is not limitedthereto.

The electrolyte solution includes a lithium salt as an electrolyte andan organic solvent for dissolving it.

The lithium salt includes those commonly used in an electrolyte solutionfor a secondary battery without limitation, and for example, an anion ofthe lithium salt may include at least one selected from the groupconsisting of F⁻, Cl⁻, I⁻, NO₃ ⁻, N(CN)₂ ⁻, BF₄ ⁻, ClO₄ ⁻, PF₆ ⁻,(CF₃)₂PF₄ ⁻, (CF₃)₃PF₃ ⁻, (CF₃)₄PF₂ ⁻, (CF₃)₅PF⁻, (CF₃)₆P⁻, CF₃SO₃ ⁻,CF₃CF₂SO₃ ⁻, (CF₃SO₂)₂N⁻, (FSO₂)₂N⁻, CF₃CF₂(CF₃)₂CO⁻, (CF₃SO₂)₂CH⁻,(SF₅)₃C⁻, (CF₃SO₂)₃C⁻, CF₃(CF₂)₇SO₃ ⁻, CF₃CO₂ ⁻, CH₃CO₂ ⁻, SCN⁻ and(CF₃CF₂SO₂)₂N⁻.

The organic solvent included in the electrolyte solution is not limitedto a particular type and may include commonly used types, and maytypically include at least one selected from the group consisting ofpropylene carbonate, ethylene carbonate, diethylcarbonate,dimethylcarbonate, ethylmethylcarbonate, methylpropylcarbonate,dipropylcarbonate, dimethylsulfoxide, acetonitrile, dimethoxyethane,diethoxyethane, vinylenecarbonate, sulfolane, γ-butyrolactone,propylenesulfite and tetrahydrofuran.

Particularly, among the carbonate-based organic solvents, cycliccarbonate such as ethylenecarbonate and propylenecarbonate is an organicsolvent with high viscosity and may be preferably used because ofallowing favorable dissolution of the lithium salt in the electrolyte byvirtue of a high dielectric constant. When such cyclic carbonate ismixed with linear carbonate, which has a low viscosity and a lowdielectric constant, such as dimethyl carbonate and diethyl carbonate ata proper ratio, it is possible to form an electrolyte solution havinghigh electrical conductivity.

Optionally, the electrolyte solution according to the present disclosuremay further include an additive such as an overcharge inhibitorconventionally included in electrolyte solutions.

The lithium secondary battery according to an embodiment of the presentdisclosure may be manufactured by interposing the separator between thepositive electrode and the negative electrode to form an electrodeassembly, putting the electrode assembly in, for example, a pouch, acylindrical battery case or a prismatic battery case, and injecting theelectrolyte. Alternatively, the electrode assembly is stacked andimpregnated with the electrolyte solution, and the obtained result isput in the battery case, which in turn, is sealed, bringing the lithiumsecondary battery into completion.

According to an embodiment of the present disclosure, the lithiumsecondary battery may be a stack type, a winding type, a stack andfolding type or a cable type.

The lithium secondary battery according to the present disclosure may beused in battery cells used as a power source of small devices, andpreferably, may be also used in medium- and large-sized devicesincluding a plurality of battery cells. Preferred examples of themedium- and large-sized devices include electric vehicles, hybridelectric vehicles, plug-in hybrid electric vehicles and energy storagesystems, and particularly, may be usefully used in hybrid electricvehicles and new renewable energy storage batteries in the arearequiring high output.

MODE FOR DISCLOSURE

Hereinafter, examples will be described in detail to provide a furtherunderstanding of the present disclosure. However, the examples accordingto the present disclosure maybe modified in many different forms andshould not be construed as limited to the examples described below. Theexamples of the present disclosure are provided to fully explain thepresent disclosure to those skilled in the art.

EXAMPLE 1 Manufacture of Negative Electrode Including Doped andComplex-Shaped Complex Particles

Step 1: Doping of SiO Particles

100 g of SiO particles having the average particle diameter D₅₀ of 1.5μm and 10 g of magnesium powder having the average particle diameter D₅₀of 5 μm are mixed and put into a chamber, and heat treatment isperformed under an Ar atmosphere at 950° C. for 2 hours to dope the SiOparticles with magnesium. In this instance, an amount of magnesium dopedin SiO is 8 weight %, and the average particle diameter of the obtainedmagnesium-doped SiO is 1.5 μm as measured using a laser diffractionparticle size analyzer (Microtrac MT 3000).

Step 2: Process of Producing Complex Particles

100 parts by weight of the SiO particles doped in the step 1, 10 partsby weight of pitch and 5 parts by weight of CNT are added to thechamber, and sintered under an Ar atmosphere at 850° C. for 3 hours,yielding complex particles in which the doped SiO particles and the CNTare formed into a complex by the medium of a carbon-based binder as aresult of sintering the pitch. The average particle diameter D₅₀ of thecomplex particles is 7 μm as measured using a laser diffraction particlesize analyzer (Microtrac MT 3000).

Step 3: Manufacture of Negative Electrode

A negative electrode active material obtained by mixing the complexparticles obtained in the above step 2 and artificial graphite at aweight ratio of 1:9, carbon black as a conductive material, andcarboxymethyl cellulose (CMC) and styrene butadiene rubber (SBR) asbinders are mixed at a weight ratio of 95.8:1:1.7:1.5.

28.9 g of distilled water is added to 5 g of the mixture to obtain aslurry for a negative electrode active material layer, and the slurry isapplied on a 20 μm thick copper thin film which is a current collector,and then dried. In this instance, in drying, the temperature ofcirculating air is 60° C. Subsequently, roll pressing is performed,followed by drying in a vacuum oven of 130° C. for 12 hours, andpunching in a round shape of 1.4875 cm² to manufacture a negativeelectrode.

EXAMPLE 2 Manufacture of Negative Electrode Including Doped andComplex-Shaped Complex Particles

Step 1: Doping of SiO Particles

100 g of SiO particles having the average particle diameter D₅₀ of 6 μmand 10 g of magnesium powder having the average particle diameter D₅₀ of5 μm are mixed and put into a chamber, and heat treatment is performedunder an Ar atmosphere at 950° C. for 2 hours to dope the SiO particleswith magnesium. In this instance, an amount of magnesium doped in SiO is8 weight %, and the average particle diameter of the obtainedmagnesium-doped SiO is 6 μm as measured using a laser diffractionparticle size analyzer (Microtrac MT 3000).

Step 2: Process of Producing Complex Particles

100 parts by weight of the SiO particles doped in the step 1, 10 partsby weight of pitch and 5 parts by weight of CNT are added to thechamber, and sintered under an Ar atmosphere at 850° C. for 3 hours,yielding complex particles in which the doped SiO particles and the CNTare formed into a complex by the medium of a carbon-based binder as aresult of sintering the pitch. The average particle diameter D₅₀ of thecomplex particles is 12 μm as measured using a laser diffractionparticle size analyzer (Microtrac MT 3000).

Step 3: Manufacture of Negative Electrode

A negative electrode is manufactured by performing the same process asthe step 3 of example 1.

COMPARATIVE EXAMPLE 1 Manufacture of Negative Electrode IncludingUndoped and Noncomplex-Shaped SiO Particles

A negative electrode active material obtained by mixing undoped SiOparticles having the average particle diameter D₅₀ of 1.5 μm andgraphite at a weight ratio of 1:9, carbon black as a conductivematerial, and carboxymethyl cellulose (CMC) and styrene butadiene rubber(SBR) as binders are mixed at a weight ratio of 95.8:1:1.7:1.5.

28.9 g of distilled water is added to 5 g of the mixture to obtain anegative electrode slurry, and the slurry is applied on a 20 μm thickcopper thin film which is a current collector, and then dried. In thisinstance, the temperature of circulating air is 60° C. Subsequently,roll pressing is performed, followed by drying in a vacuum oven of 130°C. for 12 hours, and punching in a circular shape of 1.4875 cm² tomanufacture a negative electrode.

COMPARATIVE EXAMPLE 2 Manufacture of Negative Electrode Including Dopedand Noncomplex-Shaped SiO Particles

Step 1: Doping of SiO Particles

100 g of SiO particles having the average particle diameter D₅₀ of 1.5μm and 10 g of magnesium powder having the average particle diameter D₅₀of 5 μm are mixed and put into a chamber, and heat treatment isperformed under an Ar atmosphere at 950° C. for 2 hours to dope the SiOparticles with magnesium. In this instance, an amount of magnesium dopedin SiO is 8 weight %, and the average particle diameter the obtainedmagnesium-doped SiO is 1.5 μm as measured using a laser diffractionparticle size analyzer (Microtrac MT 3000).

Step 2: Manufacture of Negative Electrode

A negative electrode is manufactured by performing the same process asthe step 3 of example 1 using the doped SiO particles.

COMPARATIVE EXAMPLE 3 Manufacture of Negative Electrode IncludingUndoped Silicon Oxide-Containing Complex Particles

Step 1: Process of Producing Complex Particles

100 parts by weight of undoped SiO particles having the average particlediameter D₅₀ of 1.5 μm, 10 parts by weight of pitch and 5 parts byweight of CNT are mixed and put into a chamber, and sintered under an Aratmosphere at 850° C. for 3 hours, yielding complex particles in whichthe undoped SiO particles and the CNT are formed into a complex by themedium of a carbon-based binder as a result of sintering the pitch. Theaverage particle diameter D₅₀ of the complex particles is 7 μm asmeasured using a laser diffraction particle size analyzer (Microtrac MT3000).

Step 2: Manufacture of Negative Electrode

A negative electrode is manufactured by performing the same process asthe step 3 of example 1 using the complex particles.

COMPARATIVE EXAMPLE 4 Manufacture of Negative Electrode IncludingComplex Particles Not Containing Linear Nonductive Material

Step 1: Doping of SiO Particles

100 g of SiO particles having the average particle diameter D₅₀ of 1.5μm and 10 g of magnesium powder having the average particle diameter D₅₀of 5 μm are mixed and put into a chamber, and the SiO particles aredoped with magnesium by heat treatment under an Ar atmosphere at 950° C.for 2 hours. In this instance, an amount of magnesium doped in SiO is 8weight %, and the average particle diameter of the obtainedmagnesium-doped SiO is 1.5 μm as measured using a laser diffractionparticle size analyzer (Microtrac MT 3000).

Step 2: Process of Producing Complex Particles

100 parts by weight of the SiO particles doped in the step 1 and 10parts by weight of pitch are added to the chamber, and sintered under anAr atmosphere at 850° C. for 3 hours, yielding complex particles inwhich the doped SiO particles are formed into a complex by acarbon-based binder as a resulting of sintering the pitch. The averageparticle diameter D₅₀ of the complex particles is 7 μm as measured usinga laser diffraction particle size analyzer (Microtrac MT 3000).

Step 3: Manufacture of Negative Electrode

A negative electrode is manufactured by performing the same process asthe step 3 of example 1 using the complex particles.

COMPARATIVE EXAMPLE 5 Manufacture of Negative Electrode IncludingComplex Particles Not Containing Sintered Carbon-Based Binder

Step 1: Doping of SiO Particles

100 g of SiO particles having the average particle diameter D₅₀ of 1.5μm and 10 g of magnesium powder having the average particle diameter D₅₀of 5 μm are mixed and put into a chamber, and heat treatment isperformed under an Ar atmosphere at 950° C. for 2 hours to dope the SiOparticles with magnesium. In this instance, an amount of magnesium dopedin SiO is 8 weight %, and the average particle diameter of the obtainedmagnesium-doped SiO is 1.5 μm as measured using a laser diffractionparticle size analyzer (Microtrac MT 3000).

Step 2: Simple Mixing Process

100 parts by weight of the SiO particles doped in the step 1, 10 partsby weight of pitch and 5 parts by weight of CNT are mixed at roomtemperature without heat treatment. In this case, it is difficult tomeasure the average particle diameter due to simple mixing.

Step 3: Manufacture of Negative Electrode

A negative electrode is manufactured by performing the same process asthe step 3 of example 1 using the mixture particles.

EXPERIMENTAL EXAMPLE Performance Evaluation of Lithium Secondary Battery

Secondary batteries are manufactured by the common method, including thenegative electrodes manufactured in examples 1 and 2 and comparativeexamples 1 to 5, and then charged/discharged. In this instance, chargingis performed by applying the current at the current density of 0.1C-rate up to the voltage of 4.2V, and discharging is performed at thesame current density up to the voltage of 2.5V. Such condition ofcharge/discharge is applied for the first cycle and initial efficiency(%) is measured in the first cycle, and capacity retention (%) ismeasured during 49 cycles under the condition of 0.5 C-rate.

Initial efficiency (%) and capacity retention (%) are calculated asbelow, and their values are shown in the following Table 1.

Initial efficiency (%)=(discharge capacity in first cycle/chargecapacity in first cycle)×100

Capacity retention (%)=(discharge capacity in 50th cycle/dischargecapacity in first cycle)×100

TABLE 1 Life characteristics Composition of Initial (capacity negativeelectrode efficiency retention %) Example 1 Doped SiO/ 87 91 linearconductive material/ sintered carbon-based binder Example 2 Doped SiO/87 87 linear conductive material/ sintered carbon-based binderComparative Undoped SiO 82 69 example 1 Comparative Doped SiO 85 76example 2 Comparative Undoped SiO/ 83 74 example 3 linear conductivematerial/ sintered carbon-based binder Comparative Doped SiO/ 86 80example 4 sintered carbon-based binder Comparative Doped SiO/ 84 81example 5 linear conductive material/ unsintered pitch

As can be seen from the above Table 1, the negative electrodemanufactured including complex particles obtained from examples 1 and 2as the negative electrode active material wherein the complex particlesare obtained through doping of SiO particles, and forming the doped SiOparticles into a complex by sintering the doped SiO particles togetherwith pitch as a carbon-based binder and CNT as a linear conductivematerial shows better initial efficiency and life characteristics thanthose of comparative examples 1 to 5 not having undergone any one of thestep 2.

1. A negative electrode active material for a lithium secondary battery,comprising: complex particles comprising: silicon oxide particles dopedwith at least one metal selected from a group consisting of lithium,magnesium, calcium and aluminum; a linear conductive material disposedbetween the doped silicon oxide particles; and a carbon-based binderthat binds the doped silicon oxide particles and the linear conductivematerial together, wherein the carbon-based binder comprises a resultantof sintering a carbon-based precursor.
 2. The negative electrode activematerial for the secondary battery according to claim 1, wherein thesilicon oxide is represented by SiO_(x), wherein 0<x≤2.
 3. The negativeelectrode active material for the lithium secondary battery according toclaim 1, wherein an amount of the metal doped in the silicon oxide is 1weight % to 50 weight % based upon a total weight of the silicon oxide.4. The negative electrode active material for the lithium secondarybattery according to claim 1, wherein the doped silicon oxide particleshave an average particle diameter of 1 μm to 6 μm.
 5. The negativeelectrode active material for the lithium secondary battery according toclaim 1, wherein the complex particles have an average particle diameterof 3 μm to 12 μm.
 6. The negative electrode active material for thelithium secondary battery according to claim 1, wherein the carbon-basedprecursor comprises pitch.
 7. The negative electrode active material forthe lithium secondary battery according to claim 1, wherein thecarbon-based binder is present in an amount of 1 part by weight to 30parts by weight based on 100 parts by weight of the doped silicon oxide.8. The negative electrode active material for the lithium secondarybattery according to claim 1, wherein the linear conductive materialcomprises one or more selected from the group consisting of carbonnanotube (CNT), graphene and carbon black.
 9. The negative electrodeactive material for the lithium secondary battery according to claim 1,wherein the linear conductive material is present in an amount of 1 partby weight to 30 parts by weight based on 100 parts by weight of thedoped silicon oxide.
 10. A method for preparing a negative electrodeactive material for a lithium secondary battery, comprising: mixingsilicon oxide particles with metal powder of at least one metal selectedfrom the group consisting of lithium, magnesium, calcium and aluminum;performing heat treatment to dope the silicon oxide particles; mixingthe doped silicon oxide particles with a carbon-based precursor and alinear conductive material; and performing sintering to form the dopedsilicon oxide particles and the linear conductive material into acomplex comprising a carbon-based binder which is a resultant ofsintering the carbon-based precursor.
 11. The method according to claim10, wherein the heat treatment in the doping is performed at atemperature of from 800° C. to 1,050° C.
 12. The method according toclaim 10, wherein the sintering to form the complex is performed at atemperature of from 700° C. to 1,100° C.
 13. A negative electrodecomprising: a current collector; and a negative electrode activematerial layer disposed on at least one surface of the currentcollector, wherein the negative electrode active material layer includesthe negative electrode active material for the lithium secondary batteryaccording to claim
 1. 14. A lithium secondary battery comprising thenegative electrode according to claim 13.